Aluminum alloy casting and method of manufacturing same

ABSTRACT

An aluminum (Al)-magnesium (Mg)-silicon (Si)-based aluminum alloy casting includes: at least boron (B) and phosphorus (P). The boron (B) and the phosphorus (P) satisfy B/P≥8 in percentage by weight.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 U.S.C. § 119to Japanese Patent Application 2020-158839, filed on Sep. 23, 2020, theentire content of which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to an Al—Mg—Si-based aluminum alloy casting anda method of manufacturing the same.

BACKGROUND DISCUSSION

In the related art, in order to increase mechanical strength ofaluminum, an Al—Mg—Si-based aluminum alloy casting in which magnesiumand silicon are added to aluminum and alloyed is known (for example, seeJP 2017-210653A (Reference 1)). It is known that when phosphorus as animpurity is contained in the aluminum alloy casting, an action ofrefining a eutectic of an Mg—Si-based compound is inhibited.

The aluminum alloy casting described in Reference 1 is subjected to adephosphorization treatment before casting to reduce a phosphoruscontent to 0.0002 wt % or less. Further, in the aluminum alloy castingdescribed in Reference 1, by setting a manganese content to 0.2 wt % to2 wt %, a fine Al—Mn—Si-based crystallized product becomes acrystallization nucleus of an Mg—Si-based crystallized product, and theMg—Si-based crystallized product is refined. Accordingly, an aluminumalloy casting having excellent toughness is obtained.

Examples of the dephosphorization treatment in the related art include amethod described in JP 2002-80920A (Reference 2). In thedephosphorization treatment method described in Reference 2, magnesiumis added to molten aluminum, and chlorine gas is blown into the moltenaluminum to float MgCl₂ that absorbs Mg₃P₂ in the molten aluminumthereby removing phosphorus from the surface of the molten aluminum.

When the dephosphorization treatment is performed before casting as inthe aluminum alloy casting described in Reference 1, it is necessary touse the dephosphorization treatment method described in Reference 2.However, since chlorine gas is extremely toxic, handling thereofrequires attention, and it is pointed out as a causative substance ofozone holes, which may lead to an environmental problem. In addition, inorder to float and remove MgCl₂ absorbing Mg₃P₂, magnesium is wasted,and a facility for detoxifying chlorine gas is additionally required,which may lead to an increase in manufacturing cost. Further, in ageneral molten metal facility, P₂O₅ is contained in a refractory binderand a coating agent of a ruddle, and phosphorus generated by P₂O₅ ismixed in the molten metal, which makes it difficult to refine theMg—Si-based crystallized product.

A need thus exists for an Al—Mg—Si-based aluminum alloy casting and amethod of manufacturing the same which are not susceptible to thedrawback mentioned above.

SUMMARY

A characteristic configuration of an aluminum alloy casting according tothis disclosure is that the aluminum alloy casting is an aluminum(AO-magnesium (Mg)-silicon-(Si)-based aluminum alloy casting includingat least boron (B) and phosphorus (P), and the boron (B) and thephosphorus (P) satisfies B/P≥8 in percentage by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional features and characteristics of thisdisclosure will become more apparent from the following detaileddescription considered with the reference to the accompanying drawings,wherein:

FIG. 1 is a diagram showing preconditions of a casting test;

FIG. 2 is an enlarged photograph of an aluminum alloy casting accordingto Example 1;

FIG. 3 is an enlarged photograph of an aluminum alloy casting accordingto Example 2;

FIG. 4 is an enlarged photograph of an aluminum alloy casting accordingto Example 3;

FIG. 5 is an enlarged photograph of an aluminum alloy casting accordingto Example 4;

FIG. 6 is an enlarged photograph of an aluminum alloy casting accordingto Example 5;

FIG. 7 is an enlarged photograph of an aluminum alloy casting accordingto Comparative Example 1;

FIG. 8 is an enlarged photograph of an aluminum alloy casting accordingto Comparative Example 2; and

FIG. 9 is an enlarged photograph of an aluminum alloy casting accordingto Comparative Example 3.

DETAILED DESCRIPTION

Hereinafter, an embodiment of an Al—Mg—Si-based aluminum alloy castingand a method of manufacturing the same according to this disclosure willbe described with reference to the drawings. However, this disclosure isnot limited to the following embodiment, and various modifications canbe made without departing from the scope of this disclosure.

The present inventors have found a composition of an aluminum alloycasting that can refine an Mg—Si-based crystallized product bydetoxifying phosphorus without performing dephosphorization treatmentbefore casting. That is, in the present embodiment, by adding boron tomake B/P≥8, phosphorus is detoxified, the Mg—Si-based crystallizedproduct is refined, and the toughness of the aluminum alloy casting canbe increased.

The aluminum alloy casting according to the present embodiment contains2.0 wt % or more and 8.0 wt % or less of magnesium (Mg), 1.0 wt % ormore and 5.0 wt % or less of silicon (Si), 0.005 wt % or more and 0.05wt % or less of boron (B), 0.0003 wt % or more and 0.005 wt % or less ofphosphorus (P), and a balance being aluminum (Al) and inevitableimpurities. The aluminum alloy casting contains boron and phosphorus soas to satisfy B/P≥8. The aluminum alloy casting may contain 0.3 wt % orless of iron (Fe), 0.3 wt % or more and 0.8 wt % or less of manganese(Mn), 0.2 wt % or less of titanium (Ti), and 0.001 wt % or more and 0.01wt % or less of beryllium (Be). At this time, titanium is preferablyadded so as to satisfy (B—8P)/Ti≥0.13.

[Composition] (Mg: 2.0 wt % or More and 8.0 wt % or Less)

Mg contributes to improvement in tensile strength of the aluminum alloycasting. When the content of Mg is less than 2.0 wt %, an amount of theMg—Si-based crystallized product decreases, and the tensile strengthdecreases. On the other hand, when the content of Mg exceeds 8.0 wt %,the amount of the Mg—Si-based crystallized product becomes excessivelylarge, and ductility decreases. When the content of Mg exceeds 8.0 wt %,remarkable oxidation of the molten metal occurs, which is thus notpreferred.

(Si: 1.0 wt % or More and 5.0 wt % or Less)

Si contributes to improvement in tensile strength of the aluminum alloycasting. When the content of Si is less than 1.0 wt %, an amount of theMg—Si-based crystallized product decreases, and the tensile strengthdecreases. When the content of Si is less than 1.0 wt %, the castability(melt flowability, seizure resistance, and casting cracking resistance)is deteriorated, which is not preferred. On the other hand, when thecontent of Si exceeds 5.0 wt %, the amount of the Mg—Si-basedcrystallized product becomes excessively large, and the ductilitydecreases.

(Fe: 0.3 wt % or Less)

Fe contributes to improvement in tensile strength of the aluminum alloycasting. When the content of Fe exceeds 0.3 wt %, an Al—Si—Fe-basedcrystallized product is generated, and the ductility decreases.

(Mn: 0.3 wt % or More and 0.8 wt % or Less)

Mn improves the baking property of a mold. When a content of Mn is lessthan 0.3 wt %, it is difficult to obtain an effect of preventing bakingof the aluminum alloy casting with respect to the mold. On the otherhand, when the content of Mn exceeds 0.8 wt %, an Al—Si—Mn-basedcrystallized product is generated, and the ductility decreases.

(B: 0.005 wt % or More and 0.05 wt % or Less)

B detoxifies phosphorus and refines the Mg—Si-based crystallizedproduct. When a content of B is less than 0.005 wt %, a function ofdetoxifying phosphorus cannot be exhibited. On the other hand, when thecontent of B exceeds 0.05 wt %, AlB-based coarse compounds crystallize,and the ductility decreases.

(P: 0.0003 wt % or More and 0.005 wt % or Less)

When a content of P is less than 0.0003 wt %, a function of refining aeutectic of the Mg—Si-based compound is inhibited. In order to make thecontent of P be 0.0003 wt % or more, a dephosphorization treatment isrequired in advance. On the other hand, when P exceeds 0.005 wt %, anAlP compound serving as a crystallization nucleus of the Mg—Si-basedcrystallized product crystallizes, and the ductility decreases.

(B/P≥8)

A ratio of boron and phosphorus in the above-described range is B/P≥8.By adding boron in the range, phosphorus is detoxified, the Mg—Si-basedcrystallized product is refined, and the toughness of the aluminum alloycasting can be increased. It is presumed that this is because a B—Pbased compound (detoxification of phosphorus), which is moreenergetically stable than an AlP compound serving as a crystallizationnucleus of the Mg—Si-based crystallized product, is generated, andcrystallization and growth of the Mg—Si-based crystallized product usingthe AlP compound as a nucleus are prevented.

In the present embodiment, boron having toxicity similar to that of saltis used. Since boron detoxifies phosphorus rather than removingphosphorus, magnesium is not wasted. Moreover, since it does not requireequipment for detoxifying chlorine gas as in the related art, it isenvironmentally friendly and the toughness of the aluminum alloy castingcan be increased at low cost. In particular, in the present embodiment,a required amount of boron for detoxifying phosphorus is defined asB/P≥8, and the phosphorus detoxifying function of boron is enabled evenif P₂O₅ is contained in a molten metal facility.

(Ti: 0.2 wt % or Less)

Ti is known as a substance that refines a primary crystal a (Al) phase.When the content of Ti exceeds 0.2 wt %, the phosphorus detoxifyingfunction of boron is inhibited. When the content of Ti exceeds 0.2 wt %,AlTi-based coarse compounds crystallize and the ductility decreases.

((B−8P)/Ti≥0.13)

In general, it is known that, when titanium and boron are simultaneouslyadded, the primary crystal a (Al) phase is refined. However, the presentinventors have found that titanium exceeding a predetermined amountinhibits the phosphorus detoxifying function of boron. Therefore, whenB/P≥8 is assumed and (B−8P)/Ti≥0.13 is satisfied, phosphorusdetoxification of boron functions, the Mg—Si-based crystallized productis refined, and the toughness of the aluminum alloy casting can befurther increased.

(Be: 0.001 wt % or More and 0.01 wt % or Less)

Be exerts an effect of preventing oxidation of the molten metal. When Beis less than 0.001 wt %, it is difficult to obtain the effect ofpreventing the oxidation of the molten metal. On the other hand, even ifBe is contained in an amount exceeding 0.01 wt %, there is no change inthe effect of preventing the oxidation of the molten metal.

[Manufacturing Method]

A method of manufacturing the aluminum alloy casting according to thepresent embodiment includes a melting step of melting a startingmaterial of the aluminum alloy casting to generate a molten metal, and acasting step of casting the molten metal generated in the melting stepwith a mold to manufacture the aluminum alloy casting. The casting stepincludes a cooling step of cooling the molten metal at 50° C./s or more.The aluminum alloy casting is used for vehicle body parts, engine parts,and the like. When the molten metal generated in the melting step iscooled at 50° C./s or more as in the present method, the Mg—Si-basedcrystallized product is subjected to homogeneous nucleation due tosupercooling, and a refining effect can be obtained. The method ofmanufacturing the aluminum alloy casting according to the presentembodiment may be gravity casting or die casting.

EXAMPLES

FIG. 1 shows a composition of the aluminum alloy casting, a ratio ofB/P, a ratio of (B−8P)/Ti, and a cooling rate of Examples andComparative Examples according to the present embodiment. The startingmaterial of the aluminum alloy casting having the composition shown inFIG. 1 was placed into a crucible and melted at a melting temperature of760° C. to 780° C. using an electric melting furnace (melting step). Atthis time, the inside of the crucible was stirred 50 times, and afterbubbling argon gas at 2 L/min for 15 minutes, the crucible was allowedto stand for 15 minutes. The molten metal generated in the melting stepwas placed into a copper mold and cast at a casting temperature of 720°C. to obtain the aluminum alloy casting (casting step). In the castingstep, the molten metal was cooled at the cooling rate shown in FIG. 1and held for a predetermined time (cooling step).

FIGS. 2 to 9 show enlarged photographs of the metal structures ofrespective aluminum alloy castings. A dark gray needle-like structure inthe photograph is the Mg—Si-based crystallized product, and the lightgray part is the aluminum base material.

In the case where Ti is not contained, in Comparative Example 1 in whichP is 0.0011 mass % and B is not added (B/P=0, (B−8P)/Ti=∞, cooling rate:50° C./s), as shown in FIG. 7, a Mg—Si-based eutectic is coarse (darkgray needle-like structure is thick and long), whereas in Example 1 inwhich 0.0010 mass % of P and 0.0080 mass % of B are added (B/P=8,(B−8P)/Ti=∞, cooling rate: 50° C./s), and in Example 2 in which 0.0011mass % of P and 0.0200 mass % of B are added (B/P=18, (B−8P)/Ti=∞,cooling rate: 50° C./s), as shown in FIGS. 2 and 3, the Mg—Si-basedeutectics are refined (dark gray needle-like structure is thin andshort). In Comparative Example 2 in which 0.0010 mass % of P and 0.0060mass % of B are added (B/P=6, (B−8P)/Ti=−∞, cooling rate: 50° C./s), asshown in FIG. 8, the Mg—Si-based eutectic is coarse. From above, it canbe understood that when B/P≥8 in the case where Ti is not contained, theMg—Si-based eutectic is refined, and when (B−8P)/Ti≥0.13 in the casewhere Ti is contained, the Mg—Si-based eutectic is refined.

In the case where Ti is contained, in Example 3 in which 0.04 mass % ofTi, 0.0017 mass % of P, and 0.0377 mass % of B are added (B/P=22,(B−8P)/Ti=0.6, cooling rate: 50° C./s), and in Example 4 in which 0.13mass % of Ti, 0.0021 mass % of P, and 0.0334 mass % of B are added(B/P=16, (B−8P)/Ti=0.13, cooling rate: 50° C./s), as shown in FIGS. 4and 5, the Mg—Si-based eutectic is refined. In Example 5 in which 0.13mass % of Ti, 0.0018 mass % of P, and 0.0147 mass % of B are added(B/P=8, (B−8P)/Ti=0.002, cooling rate: 50° C./s), coarse Mg—Si-basedeutectic is present in a very small part of the aluminum alloy casting(not shown), whereas the Mg—Si-based eutectic is roughly refined asshown in FIG. 6. From above, it is found that, in the case where Ti iscontained, when (B−8P)/Ti≥0.13, the Mg—Si-based eutectic can be reliablyrefined.

In Comparative Example 3 in which the cooling rate is as low as 10°C./s, chemical components are the same as those in Example 1 in whichthe cooling rate is 50° C./s, whereas the Mg—Si-based eutectic is coarseas shown in FIG. 9. That is, it can be understood that when the coolingrate is 50° C./s or more in the cooling step, the Mg—Si-based eutecticis refined.

INDUSTRIAL APPLICABILITY

This disclosure is applicable to an Al—Mg—Si-based aluminum alloycasting and a method of manufacturing the same.

A characteristic configuration of an aluminum alloy casting according tothis disclosure is that the aluminum alloy casting is an aluminum(Al)-magnesium (Mg)-silicon-(Si)-based aluminum alloy casting includingat least boron (B) and phosphorus (P), and the boron (B) and thephosphorus (P) satisfies B/P≥8 in percentage by weight.

The present inventors have found a composition of an aluminum alloycasting that can refine an Mg—Si-based crystallized product bydetoxifying phosphorus without performing a dephosphorization treatmentbefore casting. That is, in this configuration, by adding boron to makeB/P≥8, phosphorus is detoxified, the Mg—Si-based crystallized product isrefined, and the toughness of the aluminum alloy casting can beincreased. It is presumed that this is because a B—P-based compound(detoxification of phosphorus), which is more energetically stable thanan AlP compound serving as a crystallization nucleus of the Mg—Si-basedcrystallized product, is generated, and crystallization and growth ofthe Mg—Si-based crystallized product using the AlP compound as a nucleusare prevented.

In this configuration, boron having toxicity similar to that of salt isused. Since boron detoxifies phosphorus rather than removing phosphorus,magnesium is not wasted. Moreover, since it does not require equipmentfor detoxifying chlorine gas as in the related art, it isenvironmentally friendly and the toughness of the aluminum alloy castingcan be increased at low cost.

In particular, in this configuration, a required amount of boron fordetoxifying phosphorus is defined as B/P≥8, and a phosphorus detoxifyingfunction of boron is enabled even if P₂O₅ is contained in a molten metalfacility. As described above, it is possible to provide anAl—Mg—Si-based aluminum alloy casting which is environmentally friendlyand can increase the toughness at low cost.

Another characteristic configuration is that the aluminum alloy castingfurther includes titanium (Ti), and the boron (B), the phosphorus (P),and the titanium (Ti) satisfy (B−8P)/Ti≥0.13 in percentage by weight.

In general, it is known that, when titanium and boron are simultaneouslyadded, a primary crystal a (Al) phase is refined. However, the presentinventors have found that titanium exceeding a predetermined amountinhibits the phosphorus detoxifying function of boron. Therefore, when(B−8P)/Ti≥0.13 is satisfied as in this configuration, phosphorusdetoxification of boron functions, the Mg—Si-based crystallized productis refined, and the toughness of the aluminum alloy casting can befurther increased.

Another characteristic configuration is that the boron (B) is 0.005 wt %or more and 0.05 wt % or less, and the phosphorus (P) is 0.0003 wt % ormore and 0.005 wt % or less.

Within the range of boron as in this configuration, phosphorus isreliably detoxified, and a coarse boron compound is not formed.

Another characteristic configuration is that the magnesium (Mg) is 2.0wt % or more and 8.0 wt % or less, the silicon (Si) is 1.0 wt % or moreand 5.0 wt % or less, the titanium (Ti) is 0.2 wt % or less, and abalance includes the aluminum (Al) and inevitable impurities.

Due to the composition according to this configuration, castability(melt flowability, seizure resistance, and casting cracking resistance)and mechanical properties are excellent. Therefore, it is possible toprovide an Al—Mg—Si-based aluminum alloy casting which isenvironmentally friendly and can increase the toughness at low cost.

A method of manufacturing the aluminum alloy casting according to thisdisclosure is a method of manufacturing the aluminum alloy castingaccording to any one of the above aspects, the method including: amelting step of melting a starting material of the aluminum alloycasting to generate a molten metal; and a cooling step of cooling themolten metal generated in the melting step at 50° C./s or more.

When the molten metal generated in the melting step is cooled at 50°C./s or more as in the present method, the Mg—Si-based crystallizedproduct is subjected to homogeneous nucleation due to supercooling, anda refining effect can be obtained.

The principles, preferred embodiment and mode of operation of thepresent invention have been described in the foregoing specification.However, the invention which is intended to be protected is not to beconstrued as limited to the particular embodiments disclosed. Further,the embodiments described herein are to be regarded as illustrativerather than restrictive. Variations and changes may be made by others,and equivalents employed, without departing from the spirit of thepresent invention. Accordingly, it is expressly intended that all suchvariations, changes and equivalents which fall within the spirit andscope of the present invention as defined in the claims, be embracedthereby.

What is claimed is:
 1. An aluminum (Al)-magnesium (Mg)-silicon(Si)-based aluminum alloy casting comprising: at least boron (B) andphosphorus (P), wherein the boron (B) and the phosphorus (P) satisfyB/P≥8 in percentage by weight.
 2. The aluminum alloy casting accordingto claim 1, further comprising: titanium (Ti), wherein the boron (B),the phosphorus (P), and the titanium (Ti) satisfy (B−8P)/Ti≥0.13 inpercentage by weight.
 3. The aluminum alloy casting according to claim1, wherein the boron (B) is 0.005 wt % or more and 0.05 wt % or less,and the phosphorus (P) is 0.0003 wt % or more and 0.005 wt % or less. 4.The aluminum alloy casting according to claim 3, wherein the magnesium(Mg) is 2.0 wt % or more and 8.0 wt % or less, the silicon (Si) is 1.0wt % or more and 5.0 wt % or less, the titanium (Ti) is 0.2 wt % orless, and a balance includes the aluminum (Al) and inevitableimpurities.
 5. A method of manufacturing the aluminum alloy castingaccording to claim 1, comprising: a melting step of melting a startingmaterial of the aluminum alloy casting to generate a molten metal; and acooling step of cooling the molten metal generated in the melting stepat 50° C./s or more.